RustCrypto wishlist for v0.14

[tarcieri]

I thought I'd make a tracking issue of things I'd like to see in v0.14. For all of these at least preliminary work is done, and for many they're working and tested in our rustcrypto-group fork of this crate and downstream crates that consume it, and we'd like to find an acceptable way to get the functionality upstream so we can migrate from rustcrypto-group back to group:

• wNAF: sum-of-products multiscalar multiplication / linear combination #75
• wNAF uses PrimeField::to_repr on scalars assuming little endian #78
• wNAF: support for custom scalar multiplication algorithms #79
• Consider replacing subtle with ctutils: [WIP] rfc: replace subtle with ctutils rfcs#3

[tarcieri]

I will go ahead and close this.

#85 is open to add multiscalar multiplication.

I will need to write an RFC for endianness handling.